As packaging materials for packaging foods, industrial products and the like, laminates provided with a gas barrier layer to prevent permeation of gas have been widely used. Among these, in applications that require particularly high gas barrier properties, two gas barrier layers are provided in a laminate in some cases.
For example, Patent Document 1 discloses a packaging material in which a layer of an inorganic compound vapor deposition film on which a thin film of an inorganic compound is vapor-deposited is provided on one outermost surface of a laminate as a first gas barrier layer, and a layer formed of an aluminum foil is provided as a second gas barrier layer. In the packaging material, a layer formed of a nylon film is provided between the first gas barrier layer and the second gas barrier layer, and each of the layers is bonded to one another by a dry lamination method using an adhesive.
As adhesives used in the dry lamination method, there are aqueous adhesives and organic solvent adhesives, and among these, a solvent two-component reaction polyurethane-based adhesive is frequently used. The two-component reaction polyurethane-based adhesive includes a main ingredient (polyol component) having a hydroxyl group at the polymer terminal and a curing agent (polyisocyanate component) having an isocyanate group and is cured by forming a urethane bond by the reaction of the hydroxyl group and the isocyanate group.
However, in a case in which there is moisture in the reaction process during curing, the two-component reaction polyurethane-based adhesive generates carbon dioxide resulting from the reaction of the moisture and the isocyanate group of the curing agent. Therefore, in a case in which the two-component reaction polyurethane-based adhesive is used between the first gas barrier layer and the second gas barrier layer, there is a problem in external appearance.
That is, since both the first gas barrier layer and the second gas barrier layer have gas barrier properties, it is difficult to release the carbon dioxide generated from the adhesive to the outside air through these layers. Therefore, the carbon dioxide remains between the first gas barrier layer and the second gas barrier layer in the form of bubbles.
Then, such bubbles become visible from the outside through the transparent first gas barrier layer and a defect in the external appearance occurs. In addition, in a case in which small bubbles coalesce to form large bubbles, there is a possibility that the large bubbles may cause delamination.
Patent Document 2 discloses a technique of, in a case in which a first gas barrier layer formed of a plastic film on which a thin inorganic film is vapor-deposited, and a second gas barrier layer formed of an aluminum foil are bonded to each other with an adhesive, arranging the first gas barrier layer such that the side of the first gas barrier layer on which the thin inorganic film is not vapor-deposited is brought into contact with the adhesive. According to Patent Document 2, carbon dioxide generated from the adhesive is absorbed by the plastic film or is absorbed by the plastic film and then released from the end face of the plastic film to the outside air. Thus, the carbon dioxide does not remain in the laminate in the form of bubbles.